This invention relates to a device for maintaining apparatus therein at very low temperatures, and, more particularly, to such a device which does not utilize a large heat sink of liquefied gas.
In several branches of technology, it is necessary to maintain apparatus such as electronic devices at very cold temperatures, approaching absolute zero. In one example that is of interest presently, some electronic devices must operate at a low temperature of about 4K ("K" being the symbol for degrees Kelvin) to reduce electronic noise resulting from thermal fluctuations in circuits, and induce a superconducting state in electronic components. There are many other circumstances where the low temperature is required, as to slow the rate of chemical reactions or to achieve other physical effects.
Most commonly, a cold environment is achieved using a highly insulated container, called a dewar, filled with a cold liquefied gas. For example, nitrogen liquefies at 77K and helium liquefies at 4.2K. Where temperatures near absolute zero (0K) are needed, liquid helium is used as the coolant. A device that resides in a bath of such a liquefied gas is maintained at a temperature that is no higher than the boiling point of the liquid gas, and is often lower. At these very low temperatures, heat rapidly leaks into the liquid helium from the exterior of the container along any available thermal conductor, so that the liquefied gas boils away rapidly unless extreme care is taken to insulate the vessel against heat leaks. When such measures are taken, apparatus in the container can be maintained at such low temperatures almost indefinitely, as long as new liquid gas is periodically added to replace that which boils away.
Providing a container full of liquid helium coolant is possible in many research-oriented facilities. However, liquid helium cannot be readily supplied at other locations such as remote sites where apparatus is to be continuously operated at cryogenic temperatures. Liquid helium is relatively expensive anywhere, and may be prohibitively expensive or completely unavailable at remote sites. The conventional approach also suffers from the drawback that the container must be maintained in an upright position without tilting it too far in any direction, or the helium can leak out or be exposed to warm structure that causes it to boil rapidly. In some instances the rapid bubbling of the helium can cause erroneous measurements in some very sensitive types of devices.
Responsive to these drawbacks with the use of a bath of liquid helium as a coolant, there have been developed systems where the cooling of the interior is achieved by an approach other than a bath of liquid gas, termed a nonimmersive cooling. In one type of device, an appropriate gas such as helium is precooled and piped under pressure into the insulated container and expanded through a nozzle. As the gas expands through the nozzle, it cools and can absorb heat from its surroundings, thereby cooling them. Cryogenic temperatures can be achieved by this approach, but an external gas precooler and insulated gas lines are required.
In another approach, a mechanical precooler is built into the apparatus to precool the gas before the gas is expanded through a nozzle. Such apparatus is built in the form of a "cold finger" in which the elongated mechanical precooler is attached to a top plate of the insulated container, and the expansion cooler and device are built onto the end of the cold finger extending furthest into the container. The drawback of this type of apparatus is that mechanical precoolers vibrate, producing vibrational amplitudes of about 40-50 micrometers or more in the apparatus being cooled. This vibrational amplitude is unacceptably large for some types of sensitive instrumentation to be cooled. Attempts to reduce the vibration have been unsuccessful, and the resulting apparatus has had unacceptably high heat leakage rates and could not maintain cryogenic temperatures for extended periods.
There therefore exists a need for a cryogenic apparatus that maintains a device therein at temperatures near absolute zero for extended periods of time by a nonimmersive technique, and also imparts, at most, very low, controlled, and stable vibrational amplitudes to the device. The present invention fulfills this need, and further provides related advantages.